06 February 2014

This single-atom engine breaks the laws of physics, could drive progress in quantum computing

A new invention from Germany’s University of Mainz is not only the world’s smallest engine by an enormous margin, it may have broken a theoretical limit for engine efficiency. The device, a so-called “atomic engine,” produces power thanks to the movements of just a single atom trapped and manipulated. It’s an incredible achievement that, while not particularly useful for engineering in the short term, could revolutionize our understanding of the quantum world. Plus, it’s really neat.

Despite its size, this engine is actually patterned after one of the simplest possible engine designs, called a Carnot engine. This idea basically describes any engine that creates mechanical work out of the transfer of heat from one place to another imagine if your thermos could power a little electrical generator for an LCD temperature display, simply off the slow loss of heat to the atmosphere. This “engine” holds a single calcium ion (charged atom) trapped in a cone of electromagnetic energy called a Paul trap. At the narrow end of the cone the device applies a heating laser that adds energy to the atom’s electrons, causing them to become more repulsive to the positively-charged nucleus and orbit further out. Since the atom is squeezed so tightly at the narrow end, this expansion causes it to rush along the length of the cone toward the wide end where it meets a cooling laser.

A calcium atom, and its electron configuration. Electrons can fall into many different shells, and can move between them at times.

This is the basic heat transfer mechanism of the engine, and in terms of its function it can be thought of as broadly similar to the moving piston of a combustion engine; in this case, the atom is a reusable fuel. Gasoline is heated (combusted) and expands, doing work, before cooling and contracting again. The only real difference is that we have to keep adding more gasoline to the engine to keep the process moving, so the input of energy is chemical. Here, the atom remains mostly static and the system receives its energy via the heating laser. The heating cycle is timed to coincide with the atom’s natural resonance, so with each cycle its movements become more powerful.

The researchers actually added one more feature before publishing, one which they characterize as the equivalent of a “supercharger” for their atomic engine. When an atom is at the heating end of the cone, the system suddenly sends a pulse to intensify and contract the cone of energy that keeps the atom in line, squeezing it. This initiates one of those trademark “weird” quantum states, called a “squeezed state.” This is essentially just another way to add energy to the system, a complement to the heating laser, and causes the atom to pulse as it races toward the cooling end. Though it might seem like a small addition, the researchers claim it can supercharge the system to fully four times its normal energy efficiency.

A 4-stroke combustion engine.

That efficiency breaks long-standing theoretical limit for the efficiency of a Carnot engine, though given how limited this engine is in application, that might not be too surprising. As we’re seeing in various recent quantum experiments, most notably a recent study which alleges to have broken Newton’s Third Law, what is possible at the atomic and sub-atomic levels is not necessarily generalizable to the macroscopic world we inhabit. That doesn’t mean it can’t be useful in some way, but it does mean that the Carnot limit stands for all practical purposes. In terms of real-world applications, there aren’t too many; though tiny in its most essential elements, controlling the engine’s lasers, EM fields, and recording devices takes up most of a lab. And while its efficiency is very high, that’s still only for its size. The actual ability to do work is miniscule.

However, this engine could drive some real progress in other areas, most notably in quantum computing. The transfer of heat, often cyclically, constitutes a huge portion of the engineering challenges behind building quantum computers and quantum communications devices. The better we understand the behavior of atoms, and the better we’re able to control their behavior to such an extreme degree, the sooner we can make the quantum world start working for us.

Now read: New super accurate atomic clock could be ultra-portable thanks to chilly atoms


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